Typically, such blocks are made by filling a mould with expandable polystyrene beads (EPS beads), blowing steam through the mould space until the beads start to react and expand, then closing the steam exhaust passage and allowing the steam pressure to build up to complete the expansion and fusion process and finally exhausting steam and condensate from the mould.
In conventional single mould machines the newly formed block has to remain in the mould until the internal pressure has dropped sufficiently to allow the mould to be opened and the block to be ejected without further swelling and consequent cracking of the block. This has limited the production rate per mould and of course the size of the block has also been limited by the size of the mould.
In order to overcome or ameliorate those limitations step-by-step continuous moulding has been proposed wherein the mould is in the form of an open-ended "tunnel", one end of which is closable by a longitudinally movable ejector wall and the other end of which opens into an open-ended curing "chamber", each side wall of which is defined by a flight of a flat plate segmented chain conveyor.
In using the last mentioned apparatus the EPS beads are charged into the steam tunnel and processed much as before with a previously formed block section in the curing chamber being relied upon to close the tunnel end remote from the ejector wall. The newly formed block section fuses with the section in the curing chamber and in due course the ejector wall pushes the new section into the curing chamber, a move which is facilitated by operation of that chamber's chain conveyors.
As the block section in the curing chamber is contained against substantial expansion the time required in the steaming tunnel is lessened and by step-by-step operation there is virtually no limit to the length of block ultimately produced.
Nevertheless step-by-step continuous moulding as presently practiced has shortcomings which have reduced its acceptability in the industry.
During the steaming stage the foam and steam pressure exert a considerable force onto the end of the previously formed section in the curing chamber to which the new section has to fuse to make a continuous block. As the previous section is then still relatively soft and resilient, that force compresses the foam and in effect the end of the previous section is moved along. At low pressures, this movement is of no great consequence. However, if the foam to be produced has to be well fused, higher foam and steam pressures are required and the "give" is quite considerable. This movement or "give" causes two problems. Firstly, it creates additional volume in the steaming tunnel which has to be filled by the original quantity of beads charged into the tunnel, so producing a zone of lower density foam at each join. Secondly, the movement itself does not allow the new section to fuse properly onto the previous section and poor fusion, especially close to the walls, spongy corners and crack marks are the consequence.
A relatively new technique for enhancing the fusion of expanded EPS beads in single block moulds is the evacuation of air from the mould space prior to steaming. If about 80%-90% of the air is removed from the mould, the in-flowing steam almost immediately penetrates through the whole bead mass and much more uniform expansion and fusion is achieved.
Unfortunately, in experiments leading to the present invention it was found that if such high evacuation is used in continuous step-by-step moulding it causes the end of the previous section held in the curing chamber to snap off at the end of the conveyor chains and to be sucked back into the steaming tunnel. Lower vacuum was found to be ineffective, and thus pre-evacuation of the mould is not practical, in continuous step-by-step moulding machines as previously known.
Therefore, notwithstanding their high production capacity, continuous step-by-step machines have hitherto been restricted in their use to the production of low grade, open grain foams for general insulation and concrete cavity fill purposes.
The present invention was devised to ameliorate the above indicated disability of continuous step-by-step block production.